I looked a bit at the code to see how hard it would be to implement one of these; it looks like it would require a moderate amount of work, but not huge. definitely not trivial though.
If I had to guess, i'd say 6-10 hours of work (factoring in the thoughtful coding, some ui decisions, debugging, and discussion), but it's notoriously hard to estimate work times on things like this.

most of the work would be changes in movevalidator as far as I can tell, with maybe a few ancillary changes elsewhere. there's several different sections of code in there that would need to be modified to allow for some form of deeper blitz, and it's not quite as simple as just adding a check.

you'd need to decide which implementation(s) to use if you were gonna do something; my proposal, liberty's, or some other one. Doing multiple proposals at once is definitely more work than just oding one, but there's a lot of overlap, so if in the future you'd want want more than one of the implementations, it could be worthwhile to do all at once.

if one were to go for a proposal, there's a number of particular details to work out in relation to the ui about what makes for the best/easiest interface for it. like will it autoform a blitz path if you click deeper into enemy territory; how to decide which units are getting left behind for the pin force (under my proposal).

so i'd say the main question/process from her eis:

is there a mapmaker going to make a map, that will see sufficient use, to justify the work of implementation.? if not, wait until there is.

if so, then start ironing out the details of the behavior and how it will present through the interface.

@hepps
correct, in the second example it would leave 1 infantry attacking 1 defending infantry and the results of the first battle don't impact whether the second is allowed. so it'd be more of a sacrifice play to serve as a distraction while the blitzers go deep in that case.
code-wise, all the validation/control has to happen in the combat move phase, for combat itself units simply attack the province they're in.

in other circumstances it simply functions as a way to let massive stacks go past small blocker forces.

I don't remember how hard/feasible it is to implement; as it was long ago that I looked. you definitely have to make sure it has some validation locks in place to prevent people from undo-ing actions that were necessary to have enough units in the territory to let other units go past.

as I think about how it'd all work; I wonder if air units should be able to provide the "pin"; or if only units of the same domain (air/sea/land) should be able to provide the distraction that enables other forces to go past.
if an air unit was part of providing the pin, then you'd need to make sure it couldn't move again; code-wise it's probably a bunch easier to do if air units can't provide a pin.

aye, it would also help deal with the one blockin gship issue.
I think I was playing a lot of nwo when I was looking at the idea; as nwo has a lot of huge stacks blocked by one small thing. ofc changing that would massively change the balance on nwo.

A long time ago I was considering some sort of deep penetration mechanic;
the one I was considering would be that blitz (or super-blitz or whatever y awanna call it, since it'd be a different mechanic than the standard blitz rules) units could go past enemy defenders, but only if there's at least 1 other attacker for each defender.
so for instance, if you have 1 infantry and 2 tanks to attack with; and the defenders are 2 infantry; then one of the attacking tanks could go deeper, but the other tank would have to stay there
whereas if there was only one defending infantry, then both tanks could go past.

I don't think i'm gonna get to test that new version redrum, I uninstalled all my versions, cleared the map and saves directory, and did a fresh install of 9743 with fresh downloads of the maps and i'm still getting that error.
(I'm not gonna clutter this thread with anymore of this unrelated to ai bug, just wanted to let ya know I can't be testing that ai fix due to it)

Ok;
looking at another case to understand stuff generally, using finest logs (same savegame), I think I figured out wha'ts going on in this one and it's fine; it adjusted the enemy attackers up because it removed an unprofitable attack on caucasus from it's possible attacks stream, thus meaning those 3 inf can join in a counterattack on southern russia, right?:

while the ai's moves are reasonable; i'm just trying to understand how it's gettin some of its numbers here; like in the first section of its can it hold the territories checks I see this: (using finer logging setting)

but western and northern russia are both next to moscow, which is/will be intact and has a bunch of units. a bit above that in the logs there was
Removing territory that we can't successfully attack: Moscow, maxWin%=0.0, maxAttackers=2
so it should know that moscow will be intact and its troops available for counters.

other oddities are
Leningrad, value=30.751588092760592, averageAttackFromValue=21.22610523348412, MyAttackers=15, RemainingUnits=3
Leningrad, CanHold=true, MyDefenders=3, EnemyAttackers=1, win%=0.0, EnemyTUVSwing=-9.25, hasLandUnitRemaining=false
where, again at the least the air attackers from moscow are missing; as it has 4 air that would be in range to counterattack if the ai took leningrad.

and
Finland, value=16.73492259416393, averageAttackFromValue=6.142684580406411, MyAttackers=14, RemainingUnits=7
Finland, CanHold=true, MyDefenders=7, EnemyAttackers=3, win%=0.0, EnemyTUVSwing=-26.875, hasLandUnitRemaining=false
where I don't see which 3 attackers it could possibly be coming up with; as it's clearly using some air, but not all of the air if it's takin gfrom russia, or maybe it's takin from britain and ignoring the russian counterattack, I don't know. but if the ai did a 14 attackers on attack on finland then it wouldn't have taken leningrad, which would leave the leningrad stack available to counter. It makes me wonder more generally what the logs would show if I setup a scenario wherein province A has 8 units, and is adjacent to provinces B/C, which are hostile, and each have 6 units (let's say all units have att/def 2 and i'ts LL, and all other units are too far away to be relevant). in such a scenario it would be capable of taking either province reliably, but not both, and would not be able to hold onto either of them. maybe later i'll setup a test for that.

when it does the second round of its Check if it can hold territories checks, using the smaller list of targets it's selected, it's showin those same numbers again. I just don' tsee how it getting those values for EnemyAttackers in those cases.

@redrum
some of the numbers i've seen for the enemyattackers value look like it's underestimating the size of the counterattack. in particular it looks like it's not counting counterattackers who would come from another territory it's considering attacking.
I've a save I could put up to better show the point; or I could just copy part of the logs and explain the situation, whihc would be best?

@redrum
yep; that did it. doing show history even in an all human game caused the nullpointerexception.

back to dealing with the ai stuff! so when it's at: "Check if we should try to hold attack territories"
where does it derive the "enemyattackers" value from? what assumptions does it make when it does that derivation?
oh and I note there's two different times it does that check; one early on, and one somewhat later on with what looks like a smaller list of targets, which looks like a list culled down to the best targets.